The invention relates to the cleaning of ion sources for the generation of ions by matrix-assisted laser desorption (MALDI). Ion sources for the ionization of samples by matrix-assisted laser desorption (MALDI) are increasingly being used for the ionization of large molecules such as large biomolecules or synthetic polymers. In at least some fields of application in molecular biology and medical diagnostic research, higher and higher scanning rates are being demanded. Sample support plates nowadays usually hold 384, sometimes even 1536 sample spots for the analysis of individual samples. This analytical method involves exposing every sample with several hundred laser shots, so that between several hundred thousand and one million vaporization processes are necessary to analyze the samples from one sample support plate. In imaging mass spectrometry of histologic thin sections with high spatial resolution, many millions of such vaporization processes are carried out on one such histologic thin section on the sample support plate.
In MALDI ion sources, each bombardment of the samples, which contain large amounts of matrix substances in addition to the analyte substances, with the pulses of laser light generates a plasma cloud, from which the ions formed are then extracted by switching on an accelerating field. In some cases, the plasma cloud also contains solid or liquid spray particles from the quasi-explosion of the matrix material. The plasma cloud expands further, and some of the vaporized or sprayed material (mainly matrix substance with traces of analyte substance) is deposited on the acceleration diaphragms. After several hundred thousand shots, i.e. after the throughput of about ten sample support plates each containing 384 samples, or after the high spatial resolution analysis of about one square centimeter of a histologic thin section, visible coatings develop on these acceleration diaphragms around the apertures through which the ion beam passes. These coatings are electrical insulators; they can become electrically charged and interfere with the acceleration and focusing process for the ions. The coatings therefore have to be removed.
The matrix substances which are used for the matrix-assisted laser desorption (MALDI) sublime in the vacuum in noticeable quantities even at room temperatures. While the pre-prepared sample support plates can be kept under airtight conditions for more than a year without any detrimental effects, they cannot be left in a vacuum for several days without undergoing changes in the sample preparations. Under no circumstances must the sample support plates be subject to appreciable warming in the vacuum. Therefore, it is not possible to simply heat up the MALDI ion sources, as is usually done with electron impact ion sources.
Modern mass spectrometers are equipped with automatic feeding systems for sample support plates. They can thus also work through the night or even over the weekend with thousands of samples. However, the contamination problem prevents these automatic feeding systems from being operated at full capacity.
The method used almost exclusively until a few years ago for removing this coating has been to clean the electrodes manually after venting and opening the ion source. The cleaning is usually carried out with solvents such as ethanol or acetone. After opening the ion source housing, it is generally possible to clean the first acceleration diaphragm without removing the ion source; but even then, cleaning and restoring a good vacuum takes several hours, and after the mass spectrometer has been put into operation again it often has to be readjusted, and generally a complete recalibration of the calibration function for calculating the masses from the flight times must be carried out. If the ion source has to be removed for cleaning, the method takes even longer and requires an even more extensive adjustment.
A recent proposal (A. Holle and J. Franzen, DE 103 16 655 A1) involves using a specially designed cleaning plate, having precisely the same shape as the sample support plate, to clean the first acceleration diaphragm by spray-washing with solvent or by brushing. However, not only the first acceleration diaphragm but also more distant acceleration diaphragms are contaminated. The more distant acceleration diaphragms stay uncontaminated for much longer, but when the instrument is in operation for a long time with high throughput, they too have to be cleaned.
The patent application DE 10 2005 054 605 A1 (A. Holle and G. Przybyla) suggests cleaning with a reactive gas discharge, which can be automatically carried out by moving out the sample support plate, moving in a specially shaped electrode plate and admitting a reactant gas.
The two above-mentioned methods require that the sample support plate be removed from its mounting device in the ion source, however. This is particularly disadvantageous if the mass spectrometric imaging analysis of histologic thin sections is interrupted, because the sample support plate in the mounting device cannot be precisely repositioned in its earlier position with the necessary micrometer accuracy. This results in a displacement of unknown magnitude between the images before and after cleaning.
A simple cleaning method is therefore still being sought which allows the sample support plate to remain in its mounting device in the ion source. Automatic cleaning is sought for because increasing use of mass spectrometers by molecular biologists and medical professionals means that complications in the operation of the mass spectrometer must be avoided.